applied sciences

Article

Synthesis and Characterization ofPolystyrene-Montmorillonite NanocompositeParticles Using an Anionic-Surfactant-Modified Clayand Their Friction Performance

Chengcheng Yu ID , Yangchuan Ke *, Qingchun Deng, Shichao Lu, Jingqi Ji, Xu Hu and Yi Zhao

Nanotechnology Center of Energy Resources, CNPC Nanochemistry Key Laboratory, College of Science,China University of Petroleum, Beijing 102249, China; yccycc9999@163.com (C.Y.); qcdeng2014@163.com (Q.D.);lushichao1@126.com (S.L.); jjq8907@163.com (J.J.); HuXu910203@163.com (X.H.); dzhaoyi2015@163.com (Y.Z.)* Correspondence: kyc010@sohu.com

Received: 17 May 2018; Accepted: 4 June 2018; Published: 12 June 2018�����������������

Abstract: Polystyrene-organo-montmorillonite (PS-OMMT) nanocomposite particles were preparedvia emulsion polymerization of styrene in the presence of montmorillonite modified with an anionicsurfactant, sodium lauryl sulfonate (SLS), and its tribological properties as an additive topolyalphaolefin (PAO) were tested. The results of Fourier transform infrared spectroscopy (FT-IR),X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showed that SLS molecules residedin the montmorillonite (MMT) interlayer space. The effects of OMMT on the morphology andproperties of the nanocomposites were also investigated. Gel permeation chromatography (GPC) anddynamic light scattering (DLS) demonstrate that the presence of OMMT can effectively reduce theaverage molecular weight and average particle size of PS. XRD and transmission electron microscopy(TEM) of the PS-OMMT nanocomposites indicate that exfoliated and intercalated structures form andthat the MMT layers either are partly embedded inside the PS particles or remain on their surface.Compared with pure PS, the PS-OMMT nanocomposites possessed higher stability to thermaldecomposition and higher glass transition temperatures. Adding nanocomposite particles reducesthe friction coefficient, and thus, the antiwear properties of the PAO are significantly improved.The PS-OMMT-3 (3 wt % of OMMT based on styrene) particles have the best tribological performanceand maintained a stable, very low coefficient of friction of 0.09.

Keywords: organoclay; emulsion polymerization; nanoparticles; morphology; tribological property

1. Introduction

Recently, polymer-clay nanocomposites have attracted considerable interest from researchersdue to their tremendous properties and broad applications [1–5]. By introducing a low clay content(usually 1–10 wt %) into a polymer matrix, these nanocomposites can achieve many unique properties,such as reduced thermal diffusion coefficients, increased barrier characteristics, high heat-distortiontemperatures, decreased gas permeability and enhanced mechanical properties, compared withconventional composites or pure polymers [6–9]. The factors impacting the efficiency of thesenanocomposite materials include the aspect ratio of the clay layers, the dispersion quality and theinterfacial adhesion between the polymer matrix and the clay layers [10,11]. Unique and improvedproperties are often observed when the dispersed clay layers were less than 100 nm thick. Thus,many techniques have been evaluated to achieve homogeneous dispersions of ultrafine silicate claylayers inside polymer matrices. Layered silicates such as sodium-montmorillonite (MMT), which isan aluminosilicate mineral with sodium counterions between its layers, are one of the types of materials

Appl. Sci. 2018, 8, 964; doi:10.3390/app8060964 www.mdpi.com/journal/applsci

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most commonly used for making polymer-based nanocomposite inorganic materials because of theirhigh aspect ratios, large specific surface areas, high cation exchange capacities and excellent swellingcapacities [12,13]. In an MMT crystal structure, the two-dimensional layers are formed by the fusionof a central octahedral sheet of alumina with the tips of two external silica tetrahedra, and the sheetthickness is approximately 1.00 nm [14]. Due to the incompatibility of the hydrophilic MMT surfacewith the hydrophobic polymer matrix, the individual layers are not easily separated and dispersed inmost organic polymers [15]. Therefore, to improve the affinity of these clay layers for organic polymers,the clay layers are usually treated with surfactants to increase their hydrophobicity for the synthesis ofclay-based nanocomposites [12]. The surfactants can not only improve the compatibility between theclay layers and the polymer matrix but also enlarge the interlayer gallery spaces of the clay, which issignificant for allowing monomers to penetrate into the clay galleries and to polymerize.

Conventional MMT modifiers include quaternary ammonium cationic surfactants, from whichmany products have been commercialized over the past few decades. Chen et al. prepared an organicmontmorillonite with cetyltrimethylammonium bromide (CTAB) by a cationic exchange reaction andsynthesized exfoliated PS-MMT nanocomposites containing 5 wt % CTAB-MMT [16]. Simons andcoworkers employed a series of quaternary ammonium surfactants as clay modifiers to investigate theimpact of the chemical structure of the surfactant on the morphology of the PS-MMT nanocompositesprepared via bulk polymerization [17]. Zhu et al. prepared organic montmorillonite by anchoring twofunctionalized ammonium salts inside the MMT interlayer galleries to investigate the fire resistance ofthe respective intercalated and exfoliated PS-clay nanocomposites [18]. However, MMT modified withconventional cationic surfactants has a fundamental problem: the quaternary ammonium surfactantsdecompose at temperatures above 170–180 ◦C. Therefore, MMT modified by traditional cationicammonium surfactants is not suitable for high-temperature processes and applications. For thesereasons, MMT modified by an anionic surfactant was designed and prepared due to the excellentthermal stability of anionic surfactants compared to traditional cationic surfactants [19]. Furthermore,the swelling, thixotropy and dispersion properties of MMT modified by an anionic surfactant aresuperior to those of CTAB-modified MMT [19]. Nevertheless, reports on anionic surfactant-modifiedMMT account for <20% of all reported MMTs, and only a few reports have considered MMTmodification using anionic surfactants for the synthesis of polymer-clay nanocomposites [20].

Polystyrene (PS) is a thermoplastic material that is suitable for versatile applications in theengineering and coatings fields [21]. However, the thermal stability of PS is a key factor in determiningits suitability for a particular application. Layered silicates possess relatively high thermal stabilities.Therefore, the formation of PS and MMT nanocomposites could compensate for the relativelylow stability of PS such that the resulting material could have more extensive applications inhigh-temperature processes. Several methods have been applied for the preparation of polymer-claynanocomposites, such as bulk polymerization, solution polymerization, emulsion polymerizationand the melt-intercalation method [22–25]. Among these methods, emulsion polymerization wasconsidered the most promising technique for the fabrication of monodispersed polymer nanoparticleswith controlled molecular weights, faster reaction rates, and improved environmental friendlinessbecause water is used as the reaction solvent. Moreover, emulsion polymerization facilitates thepreparation of exfoliated structures in polymer-MMT composites. Zhang et al. prepared mono-dispersesilica-polymer core-shell microspheres via emulsion polymerization [26]. In addition, Li et al.synthesized exfoliated PS-MMT nanocomposites by emulsion polymerization using zwitterions asthe clay modifier, and these nanocomposites showed improvements in thermal stability comparedwith intercalated composites and the neat polymer [27]. Wang et al. observed that ultrasound orultrasonic irradiation processes assisted emulsion polymerization and could promote the formation ofexfoliated layers during nanocomposite synthesis [28]. In addition, the results of Yang et al. indicatedthat PS-clay nanocomposites synthesized via emulsion polymerization had better layer dispersionthan those synthesized by suspension polymerization [29]. Therefore, emulsion polymerization is

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an efficient technique for preparing polymer-MMT nanocomposite particles with exfoliated structuresand monodispersity.

In this paper, we report the effect of anionic surfactant modifiers on MMT and discussa possible mechanism of the effect of the anionic surfactant in the intercalated MMT. The PS-OMMTnanocomposite particles were prepared via emulsion polymerization, and the effects of OMMT on themorphology, structure, and thermal properties of the polystyrene matrix were investigated. In addition,the tribological properties of PS-OMMT nanocomposite particles as additives in polyalphaolefin (PAO)were investigated. The results showed that these nanocomposite particles were effective frictionreduction and antiwear materials and could have applications in oil and gas drilling engineering toimprove drilling fluid lubrication. Further work in this direction is currently in progress.

2. Materials and Methods

2.1. Materials

Sodium montmorillonite (MMT) with a cationic exchange capacity (CEC) of 100 meq/100 g of claywas purchased from Huai An Saibei Technology Co. Ltd. (Huaian, China). Styrene (St, >98%) monomerand divinylbenzene (DVB, >80%) were obtained from the Tianjin Guangfu Fine Chemicals ResearchInstitute (Tianjin, China) and were distilled under reduced pressure before use. Sodium lauryl sulfonate(SLS, 99%, AR) and potassium persulfate (KPS, 99%, AR) were purchased from Beijing ChemicalReagents Company (China). Polyalphaolefin (PAO, 99% purity) was purchased from Shanghai FoxChemical Technology (China). All other reagents were of analytical grade and were purchased from theTianjin Guangfu Fine Chemicals Research Institute (China), and were used without further purification.Deionized water was used throughout the experiments.

2.2. Preparation of OMMT

OMMT was prepared by a modified process based on a typical purification and separationprocess [30]. The OMMT preparation process is depicted in Figure 1 (Stage A). For the preparationof OMMT, MMT (2.5 g) was dispersed in 75.0 mL of deionized water in a three-neck flask (500 mL),and the dispersion was stirred vigorously for 30 min at room temperature. The pH of the suspensionwas adjusted to 1 using a certain amount of hydrochloric acid, and the mixture was heated to 80 ◦C for30 min with vigorous stirring. Then, a solution of SLS in 25.0 mL of deionized water was slowly addedto the clay suspension under continuous stirring. The amount of SLS was equivalent to 1.2 CEC ofMMT. After reacting for 9 h, the resulting sample was separated by filtration and thoroughly washedwith boiling deionized water until the pH of the supernatant solution was approximately 7. The finalproducts were then dried overnight at 60 ◦C in a vacuum oven, ground with a mortar and pestle,and sieved with the 200-mesh sieve for further use.

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Figure 1. Proposed reaction mechanism for the synthesis of PS-OMMT nanocomposites.

2.3. Preparation of PS-OMMT Nanocomposites

Emulsion polymerization was applied to the synthesis of the PS-OMMT nanocomposite particlesvia a previously reported procedure with slight modifications [31]. As shown in Figure 1 (Stages B,C),the surfactant SLS (0.36 g) was dissolved in 100.0 mL of deionized water in a 500 mL three-neck flask atroom temperature with continuous stirring. Thereafter, a certain amount of OMMT was dispersed inthe mixture of styrene (20.0 mL) and divinylbenzene (2.0 mL) at room temperature for 1 h to allow theOMMT to become completely swollen. Then, the suspension of OMMT was added to the reactor withcontinuous stirring at 75 ◦C for 30 min to ensure homogenization of the inorganic and organic phasesunder a nitrogen atmosphere. Finally, KPS initiator (0.072 g) was added to the mixture, and the systemwas reacted with stirring for 8 h. After the end of the reaction, the demulsification process was carriedout with ethanol, and the obtained samples were washed and purified three times using excess hotethanol to remove any unreacted monomer and oligomers, followed by filtration. The final productswere dried under reduced pressure in a vacuum oven at 60 ◦C for 24 h, prior to characterization.The nanocomposites prepared with 1, 3, and 5 wt % (based on the weight of styrene) OMMT loadingswere designated PS-OMMT-1, PS-OMMT-3, and PS-OMMT-5, respectively. For comparison, pure PSwas synthesized according to the method described above.

2.4. Preparation of PAO-PS-OMMT Lubricant

To test the tribological performance of the PS-OMMT nanocomposite particles as additives,1 wt % (based on the weight of PAO) of PS-OMMT samples were dispersed in PAO with rapid stirring.After that, the mixture was ultrasonicated for 1 h to form a homogeneous solution in PAO.

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2.5. Characterization

Fourier transform infrared (FTIR) spectra of the samples were obtained using an FTS-3000spectrophotometer (Digilab, Cambridge, MA, USA) within the range of 4000–400 cm−1 at roomtemperature. The powder samples were characterized by a KBr pressed disk technique with a weightratio of the sample to KBr of 1/100.

X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance X-ray Diffractometer(STADI P, Karlsruhe, Germany) with Cu-Ka radiation (λ = 0.1540 nm) at a scan rate of 2◦/min from1.5◦ to 20◦ with a step distance of 0.02◦. The basal spacing reflections of the samples were calculatedfrom Bragg’s Law, and the voltage and current of the X-ray tubes were 40 kV and 30 mA, respectively.

The molecular weight and molecular weight distribution were characterized by a gel permeationchromatography system (GPC515-2410, Waters, Milford, MA, USA) equipped with a 2410 RefractiveIndex Detector and a 996 Photodiode Array Detector using tetrahydrofuran (THF) as an eluent at a flowrate of 1.0 mL/min. The molecular weight calibration curve was obtained using polystyrene standards.

The particle size and size distribution of the samples were determined by dynamic light scatteringusing a Malvern Mastersizer 2000 (Malvern Instruments, Malvern, UK) at 25 ◦C.

Scanning electron microscopy (SEM, SU8010, HITACHI, Tokyo, Japan) was used to observe thesurface morphology of the samples with an accelerating voltage of 15 kV after coating the sampleswith thin layers of gold.

The dispersion morphology of the clay layers in the polymer matrix was obtained using transmissionelectron microscopy (TEM, JEM-2100, Tokyo, Japan) with an accelerating voltage of 200 kV.

The weight losses of the samples were determined using a NETZSCH thermogravimetric analyzer(TGA, STA409PC, Bavaria, Germany). The samples were between 2.0 and 5.0 mg and were heatedfrom 25 ◦C to 800 ◦C at a heating rate of 10 ◦C/min under N2 at a flow rate of 140 cm3/min.

The glass transition temperature of PS/OMMT was obtained by using differential scanningcalorimetry (DSC, 6220, Japan) at a heating rate of 10 ◦C/min under N2 at a flow rate of 100 cm3/min.

The tribological properties of the PS/OMMT samples were tested using an MRS-1J four-ball tester.GCr15 steel bearing balls with a 12.7 mm diameter (grade 10) were cleaned ultrasonically with ethanolbefore the test. All the tests were performed at a rotating speed of 1450 r/min for 60 min under a loadof 392 N at room temperature. The coefficient of friction (COF) was recorded three times, and theaverage COF values were calculated.

3. Result and Discussion

3.1. Fourier Transform Infrared Spectroscopy

Figure 2 shows the infrared spectra of MMT (A), OMMT (B), PS (C) and PS-OMMT-3 (D).In Figure 2A, for the MMT sample, the absorption bands at 3446.4 cm−1 and 1643.4 cm−1 correspondto -OH stretching vibrations and H-O-H bending vibrations of the MMT interlayer water molecules,respectively. The characteristic adsorption bands at 3632.5 cm−1 and 1036.1 cm−1 were attributed tothe Al (Mg)-OH bending vibrations and Si-O stretching vibrations, respectively. The characteristicband at 798.6 cm−1 corresponds to the Si-O-Al stretching vibrations. In contrast, for the OMMT samplespectrum shown in Figure 2B, the characteristic adsorption bands at 2925.6 cm−1 and 2860.2 cm−1

respectively correspond to the asymmetric -CH2 and symmetric -CH2 stretching vibrations in the SLSmolecular chain. In addition, the adsorption band at 1185.5 cm−1 corresponds to an -SO3H asymmetricstretching vibration. Moreover, compared with those of MMT, the absorption bands at 3440.3 cm−1

and 1632.7 cm−1 of the OMMT weaken and shift to a lower wavenumber, which indicates that thewater content in OMMT is reduced with the SLS modification. These observations indicate that theanionic surfactant molecules are present on the surface or intercalated into the interlayer spaces ofthe MMT. In Figure 2C, PS showed strong bands at 3027.9–3100.5 cm−1 and 2850.3–2920.6 cm−1,corresponding to CH aromatic stretching vibrations and CH2 asymmetric stretching vibrations,respectively. There are two absorption bands at 1603.5 cm−1 and 1447.1–1503.4 cm−1 that correspond

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to absorptions from aromatic C=C stretching vibrations. The adsorption bands at 758.7 cm−1 and698.1 cm−1 represent the C–H out-of-plane bending vibrations. In addition, the absorption band at3449.2 cm−1 corresponds to an absorption from O–H stretching vibrations, indicating the presenceof hydroxyl groups. These observations suggest that the polymerization reaction of St has occurred.Compared with PS, PS-OMMT-3 clearly shows a characteristic adsorption band at 1043.9 cm−1,which corresponds to the Si-O stretching vibrations in OMMT. The FTIR spectrum of PS-OMMT-3contains the adsorption band corresponding to OMMT, confirming the inclusion of montmorillonite inthe PS matrix.

Figure 2. FTIR spectra of (A) MMT, (B) OMMT, (C) PS and (D) PS-OMMT-3.

3.2. X-Ray Diffraction

The XRD patterns of MMT, OMMT, PS, PS-OMMT-1, PS-OMMT-3 and PS-OMMT-5 are shownin Figure 3A–F, respectively. As shown in Figure 3A, a typical MMT diffraction peak appearedat 2θ = 7.28◦, corresponding to a basal interlayer distance of 1.21 nm according to Bragg’s law.After modification by SLS, as shown in Figure 3B, the diffraction peak of OMMT shifted to 2θ = 5.79◦,suggesting that the interlayer distance had increased to 1.52 nm. This result indicated that the anionicsurfactant molecules were successfully intercalated into the MMT interlayer spaces, thereby expandingthe interlayer distance and causing the surface properties of MMT to change from hydrophilicto hydrophobic.

Figure 3. X-ray diffraction patterns of (A) MMT, (B) OMMT, (C) PS, (D) PS-OMMT-1, (E) PS-OMMT-3and (F) PS-OMMT-5.

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However, there is currently no universal understanding of the mechanism for the intercalation ofan anionic surfactant into MMT. Zhang proposed that the intercalation of the anionic surfactant wasdue to the formation of ion pairs between the surfactant anions and hydronium ions or Na+ and Ca2+

counterions and that these ion pairs then entered the interlayer spaces in different arrangements [19].However, the isomorphous substitution of Mg2+ for Al3+ or, rarely, Al3+ for Si4+ generates a netnegative charge on the surface of MMT layers [32]. Furthermore, many hydroxyl groups are presentat the edges of MMT, and few are present on the external and internal surfaces due to electrostaticinteractions with hydronium ions. In addition, in acidic media, the negative charge density on the MMTsurface will decrease, and the number of hydroxyl groups will increase because a large concentrationof hydronium ions enter the interlayer space of the MMT [19]. Therefore, the anionic surfactant couldgenerally intercalate into the MMT interlayer spaces through the replacement of or interaction withhydroxyl groups at the MMT layer edges (see stage A in Figure 1). As surfactants are inserted into theclay layers, the electrostatic repulsion between the layers gradually increases, resulting in an increasein the spacing between the clay layers. Thus, the anionic surfactant transforms MMT to OMMT withenlarged interlayer spacing, into which the styrene monomer can easily enter and polymerize.

As shown in Figure 3C–F, the disappearance of the MMT diffraction peaks in these patternsindicated that the PS chains were inserted into the OMMT gallery, which is the interlayer space ofOMMT, and destroyed the ordered clay structure during the emulsion polymerization process. In thePS-OMMT nanocomposites, no OMMT diffraction peaks were visible because the OMMT tactoidswere likely exfoliated and dispersed in a disorderly fashion into the PS matrix at the molecular levelor because the clay interlayer distances were greater than 6 nm [33]. However, the absence of XRDpeaks does not necessarily indicate complete exfoliation of the clay layers [34]. Thus, the dispersionand degree of exfoliation of the clay layers, as well as the microstructure morphology in the matrix,were further corroborated by TEM analysis. TEM micrographs of the PS and nanocomposite samplesare shown in Figure 4.

3.3. Transmission Electron Microscopy

In the TEM images, the dark lines represent the clay layers, and the brighter regions representthe polymer matrix. Figure 4A shows that the pure PS particles were transparent spheres withdiameters of approximately 130 nm. In contrast, in Figure 4B–D, the clay layers appear to havean exfoliated and/or intercalated arrangement with good dispersion in the polymer matrix. Meanwhile,in Figure 4B,C, mainly exfoliated structures (marked by yellow arrows) were seen in the PS matrix,indicating a high degree of exfoliation and dispersion of the clay layers in the matrix. Furthermore,some individual clay layers appeared to be either located on the surface of the PS particles or partiallyembedded inside the PS particles as the OMMT load increased to 3 wt %. This morphology was alsoobserved by other researchers [35]. In appropriate amounts (1–3 wt %), the OMMT can be effectivelyexfoliated to form individual layers due to the strong interactions between the OMMT and the polymermatrix. However, with an increase in the clay content, many intercalation structures (marked by redellipses) were observed, as shown in Figure 4D. This result suggested that excess OMMT can lead toineffective exfoliation and the appearance of heterogeneous stacks of clay layers due to adhesion of thenanostructures. Furthermore, all the PS-OMMT nanocomposite samples appeared to have particlesizes smaller than pure PS. This decrease in the size of the nanocomposite particles can be attributed tothe increased dispersion of clay in the PS matrix and the increase in SLS concentration, resulting in theformation of more surfactant micelles.

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Figure 4. TEM images of (A) PS, (B) PS-OMMT-1, (C) PS-OMMT-3 and (D) PS-OMMT-5.

3.4. Gel Permeation Chromatography

The molecular weight data and the impact of OMMT on the molecular weight of the PS-OMMTnanocomposite samples are summarized in Table 1.

From Table 1, it can be seen that the number-average molecular weight (Mn) and theweight-average molecular weight (Mw) of the PS-OMMT nanocomposite samples decreasedsignificantly compared with those of pure PS. This result was attributed to the MMT layers hinderingdiffusion of the monomer and/or polymer chain growth [36–38]. Interestingly, both the lowestmolecular weight and polydispersity index (PDI) values were seen in the PS-OMMT-3 sample.This result was ascribed to the better exfoliation and higher dispersion of clay layers in the PS-OMMT-3sample, leading to an increase in the diffusion path of the monomer and a decrease in polymer chaingrowth. However, as the clay content was increased, the Mn, Mw and PDI of the PS-OMMT-5 sampleincreased. This result is attributed to the reduction in the exfoliated structures and the increase in thenumber of agglomerates in the PS-OMMT-5 samples, resulting in a poor barrier effect of the clay layersand thus favoring polymer chain growth and increase of the molecular weights.

Table 1. Molecular weight distribution of PS and PS-OMMT nanocomposites.

Sample Code Mn (×105) Mw (×105) PDI

PS 3.65 15.09 4.14PS-OMMT-1 1.76 7.56 4.31PS-OMMT-3 1.56 5.57 3.57PS-OMMT-5 1.55 7.35 4.74

3.5. Thermal Stability

Thermogravimetric analysis (TGA) measurements of the MMT and OMMT samples are shownin Figure 5. As shown in Figure 5, at temperatures below 100 ◦C, the mass losses of the MMT andOMMT samples arising from the dehydration of water molecules adsorbed on the clay surfaces andthe interlayer spaces were 7.1% and 5.0%, respectively. Moreover, the water content of OMMT was lessthan that of MMT, which indicated that the absorbed water molecules were partially replaced by the

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anionic surfactant. During this stage, the SLS molecules were intercalated into the interlayer spacesof the MMT, resulting in a change from a hydrophilic MMT surface to a hydrophobic one. The MMTmass loss in the temperature range of 100–800 ◦C, corresponding to dehydroxylation of the MMT,was merely 5.3%, [34]. The total MMT mass loss was approximately 12.4%. In contrast, there wasan obvious mass loss in the temperature range of 350–500 ◦C for the OMMT samples, which wasattributed to the evaporation/decomposition of the loaded anionic surfactants, such that the mass lossreached approximately 11.4%. In the temperature range of 500–800 ◦C, the OMMT mass loss arisingfrom dehydroxylation of the MMT layers was approximately 5.3% [34]. The total mass loss of OMMTwas approximately 24.0%. As shown above, the total amount of anionic surfactants in the OMMT wasclose to 70% of mole percent based on the CEC value (i.e., 100 mmol/100 g clay). This result furtherconfirmed that the anionic surfactant was indeed intercalated into the MMT interlayers, as previouslyindicated by the XRD patterns shown in Figure 3.

Figure 5. TGA curves of (A) MMT and (B) OMMT.

Figure 6 presents TGA traces and derivative thermogravimetry (DTG) results for the pure PS andPS-OMMT nanocomposites. In Figure 6, the nanocomposites showed similar degradation behaviors(curve patterns) as the pure polymer, indicating a similar degradation mechanism to pure PS. Figure 6shows that the onset degradation temperature (T10, 10 wt % mass loss) increased from 346.1 ◦Cfor pure PS to 348.7 ◦C, 376.2 ◦C and 372.8 ◦C for the PS-OMMT-1, PS-OMMT-3 and PS-OMMT-5samples, respectively. The temperatures of the maximum decomposition rate (Tmax), as revealed bythe DTG curves, followed the same trend and increased by 6.7 ◦C, 18.1 ◦C and 13.0 ◦C, respectively,compared with pure PS. These results suggested that the introduction of OMMT improved the thermalstability of the polystyrene, which may be attributed to a labyrinth or barrier effect originating fromthe high aspect ratio of the clay layers, which can prevent diffusion of the volatile decompositionproducts out of the polymer in the thermal degradation zone [39]. However, PS-OMMT-3 was found tohave the best thermal stability and a 30.1 ◦C enhancement in the onset degradation temperature overpure PS. Furthermore, the best enhancement in the Tmax was also achieved in the PS-OMMT-3 sample.These results were attributed to the higher degree of exfoliation of the clay layers and better dispersionobtained in the PS-OMMT-3 sample than the other samples. It was plausible that the exfoliated anddispersed clay layers played a significant role as nucleating agents in the polymer matrix by limitingthe heat transfer, which may enhance the degradation temperature [31]. In addition, the improvementsin thermal stability are related to the relatively strong interfacial interactions between the clay layersand the polymer chains for the PS-OMMT-3 sample due to existence of a large number of OMMT

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single layers [40]. Meanwhile, the insufficient number of clay layers caused the presence of a barrierat low OMMT loadings (PS-OMMT-1) that could act as thermal insulators and enhance the thermalproperties of the materials, although exfoliated structures are favorable at low OMMT contents.However, a slight decrease in T10 and Tmax was observed in the PS-OMMT-5 sample. Excessive OMMTaggregation might have destroyed the effective dispersion and interaction between the clay layersand the PS matrix, resulting in a lower thermal stability [41]. Therefore, the thermal stability of thePS-OMMT nanocomposite was not only dependent on the clay loading but also closely related to theclay dispersion and degree of exfoliation, and the interactions between the OMMT and PS matrix alsoplayed an important role in improving the thermal performance [42,43].

Figure 6. TGA and DTG curves of PS, PS-OMMT-1, PS-OMMT-3 and PS-OMMT-5.

3.6. Differential Scanning Calorimetry

The thermal behaviors of pure PS and PS-OMMT nanocomposites were further investigated withDSC (Figure 7).

As shown in Figure 7, the glass transition temperature (Tg) of all the nanocomposites increasedrelative to the pure PS. This fact indicated that the clay layers can improve the Tg of the PS matrixby limiting the segmental motion of the polymer chains. PS-OMMT-3 showed the optimal increasein Tg, increasing by 3.9 ◦C compared to pure PS (124.7 ◦C). This was due to the higher degree ofexfoliation and better dispersion of the clay layers, which could effectively confine the thermal motionof the polymer chains [44]. However, at a 5 wt % OMMT loading, the Tg values of the samples woulddecrease slightly, probably due to the formation of more clay agglomerates that could not effectivelyimpede the movement of the PS chains.

Figure 7. DSC curves of (A) PS, (B) PS-OMMT-1, (C) PS-OMMT-3 and (D) PS-OMMT-5.

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3.7. Particle Size Analysis

The particle sizes and size distributions of the pure PS and its nanocomposite samples are shownin Figure 8. Table 2 summarizes the average particle size data and PDI values. As shown in Figure 8,for pure PS, a unimodal and relatively broad particle size distribution was observed, and the averageparticle diameter was approximately 129.5 nm. However, in the case of the PS-OMMT nanocompositesamples, the particle distributions were narrower, and the average particle diameters were in the rangeof 62.83–64.58 nm. The PDI values of PS, PS-OMMT-1, PS-OMMT-3 and PS-OMMT-5 samples were0.204, 0.082, 0.079 and 0.102, respectively. The particle sizes of pure PS and the nanocomposite sampleswere also confirmed by the TEM images, as shown in Figure 4. These results indicated that a certainOMMT load (<5 wt %) could decrease the particle size diameter and improve the monodispersity ofthe particles. The OMMT layers might act as nucleating agents to form smaller and more uniformnanoparticles during the polymerization process [45]. However, the minimum average particle sizeand PDI values were achieved in the samples containing 3 wt % OMMT loading. This was because ofthe higher degree of exfoliation and better dispersion of the clay layers in PS-OMMT-3 could moreeffectively impede the movement of the styrene monomers and increase the concentration of SLSto form the finer and more stable emulsion, which lead to reduction in the size of nanocompositeparticles. As shown in Figure 8 and Table 2, however, the difference in the particle size of the three claycontaining samples is not significant. In other words, the three clay containing samples are equal withrespect to particle size, considering the error in the measurement.

Figure 8. Particle size distribution curves of PS, PS-OMMT-1, PS-OMMT-3 and PS-OMMT-5.

Table 2. Particle size distribution of PS and PS-OMMT nanocomposites.

Sample Code Z-Average (nm) PDI

PS 129.5 ± 6.5 0.204PS-OMMT-1 64.58 ± 2.91 0.082PS-OMMT-3 62.83 ± 2.19 0.079PS-OMMT-5 63.38 ± 2.92 0.102

3.8. Scanning Electron Microscopy

Direct observations of the size and size distribution of the nanocomposites were made viaSEM analysis. As observed in Figure 9A, the pure PS particles were spherical with a very smoothsurface and a particle size diameter of approximately 120 to 140 nm. In contrast, the PS-OMMTnanocomposites (Figure 9B–D) showed spherical particles with obviously decreased diameters ofapproximately 60 to 70 nm. These results were in agreement with the results shown Figures 4 and 8.Furthermore, relatively smooth particle surfaces were obtained, and a certain degree of adhesion

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between these particles was also observed due to the MMT layers located either on the surface orpartially embedded inside the PS particles, as shown in Figure 4. This particle morphology indicatedthat the OMMT played a significant role in the formation of nucleation centers during the process ofemulsion polymerization, thereby restricting particle growth. However, as the amount of clay increases(above 5 wt %), the clay layers agglomerate and form intercalated structures, leading to an increase inthe particle size and PDI.

Figure 9. SEM images of (A) PS, (B) PS-OMMT-1, (C) PS-OMMT-3 and (D) PS-OMMT-5.

3.9. Friction Performance Analysis

The coefficient of friction (COF) versus the experiment time for PAO and PAO with 1 wt %(based on the weight of PAO) of the PS-OMMT nanocomposite samples is shown in Figure 10.

In Figure 10, the COF of PAO initially was approximately 0.10. However, it subsequently increasedto approximately 0.14 within 500 s and then experienced a stable period of time between approximately500 s and 700 s. The COF then decreased slightly from 0.14 to approximately 0.13 within 2500 s,followed by an increase to 0.15 with fluctuations. However, for the PAO-PS-OMMT nanocomposites,the COF values were clearly decreased compared with those of PAO because when the PS-OMMTnanocomposite particles were taken into the contact area by the lubricant, they were likely to supportthe two counterparts and prevent them from contacting each other and rolling like bearing ball inbearing between two metal surfaces to reduce the COF value. For the PAO-PS-OMMT-1 sample,the COF increased with fluctuations from 0.08 at the beginning to 0.11 until approximately 2000 s andthen remained relative steady for the rest of the test. For the PAO-PS-OMMT-3 sample, the COF startedat 0.078 and increased to approximately 0.095 within 500 s. It then remained relatively stable, and at theend of the test, the COF was approximately 0.09. For the PAO-PS-OMMT-5 sample, the COF increasedsharply from 0.083 to approximately 0.10 near 300 s and then increased slowly to approximately0.12 at the end of the test. It could be found that with the same content of nanocomposite particles,PAO-PS-OMMT-3 demonstrated the best friction reduction and the lowest COF value. This resultindicated that the PS-OMMT-3 nanocomposite particles with a higher degree of exfoliation and betterdispersion of MMT can effectively support the two counterparts and prevent contact more efficientlythan the other studied materials.

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Figure 10. The COF versus time of PAO and PAO-PS-OMMT nanocomposites.

To further investigate the variations in the tribological behavior due to the addition of thePS-OMMT nanocomposite particles, the wear surface morphologies of PAO and the nanocompositeswere examined by SEM. SEM images of the wear surfaces of the upper and lower ball of thefour-ball machine for PAO, PAO-PS-OMMT-1, PAO-PS-OMMT-3 and PAO-PS-OMMT-5 are shown inFigure 11A–D and Figure 12A–D, respectively. As shown in Figure 11A, a deep and wide groove formedon the wear surface and can be associated with the effect of the presence of hard particles between thecontact surfaces. Figure 12A further verified the existence of this worn morphology. It can be seen fromFigure 11B that the grooves on the wear surface were shallow in depth and narrow in width comparedwith those in Figure 11A, suggesting that the addition of PS-OMMT-1 nanocomposite particles couldimprove the surface roughness and reduce the COF. A smooth wear surface with microgrooves andnanogrooves was observed for the PAO-PS-OMMT-3 sample, as shown in Figures 11C and 12C.However, the wear surface of the nanocomposites containing 5 wt % OMMT show deep scratch-likelines and coarse surfaces, which indicates that the plowing phenomenon occurred more severelyon the surface compared to the composite containing 3 wt % OMMT. This result was due to theaggregation of the OMMT layers in the PS-OMMT-5 samples, which led to poor dispersion andcaused three-body abrasion, reducing the lubrication effect of the nanocomposite particles. As shownin Figure 12, the wear scar diameters (WSDs) for the PAO, PAO-PS-OMMT-1, PAO-PS-OMMT-3and PAO-PS-OMMT-5 samples were 958.14 ± 47.91 µm, 993.70 ± 44.71 µm, 929.00 ± 32.51 µm and1600.00 ± 62.40 µm, respectively. This result revealed that the PS-OMMT-3 nanocomposite particlesshowed the best antiwear properties among all the types of nanocomposite particles in this study.PS-OMMT-3 particles with better exfoliation and MMT dispersion might roll better between twosurfaces and reduce the abrasive effect of the nanocomposite particles. Furthermore, the PS-OMMT-3nanocomposite particles might fill the microgrooves more effectively and form protective films moreeasily on the metal surfaces, reducing friction more than other samples. These results suggested thatthe PS-OMMT nanocomposite particles were effective friction reduction and antiwear materials andcould find applications in drilling fluid lubrication.

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Figure 11. SEM images of the upper ball of (A) PAO-PS, (B) PAO-PS-OMMT-1, (C) PAO-PS-OMMT-3and (D) PAO-PS-OMMT-5.

Figure 12. SEM images of the lower ball of (A) PAO-PS, (B) PAO-PS-OMMT-1, (C) PAO-PS-OMMT-3and (D) PAO-PS-OMMT-5.

4. Conclusions

PS-OMMT nanocomposite particles were successfully prepared using styrene and anionicsurfactant-modified montmorillonite via emulsion polymerization, and the tribological properties ofnanocomposite particles as additives to PAO were tested. FTIR, XRD, and TGA data confirmed theintercalation of SLS into the MMT interlayer spaces in acidic media. The average molecular weightand average particle size of the PS-OMMT nanocomposite particles were significantly decreased in

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comparison with those of pure PS due to a hindering effect imposed by the OMMT layers. The XRD andTEM results for the PS-OMMT nanocomposites indicated the formation of exfoliated and intercalatedstructures and that the MMT layers were either partially embedded inside the PS particles or locatedon their surface. The TGA and DSC traces of the PS-OMMT nanocomposite particles revealed that theyexhibited enhanced thermal decomposition stability and glass transition temperatures relative to purePS. The PAO systems incorporating the nanocomposite particles exhibited higher friction reductionand antiwear properties compared with pure PAO. In addition, the nanocomposite particles at 3%OMMT-loading presented the best lubrication performance and kept a stable but very low COF levelof 0.09. The proposed mechanism for all these improvements involved the effects of exfoliation anddispersion of the OMMT, as well as the stronger interaction between the OMMT and the PS matrix.

Author Contributions: Data curation, Q.D. and S.L.; Investigation, C.Y.; Methodology, Y.K.; Software, Y.Z.;Writing—original draft, C.Y.; Writing—review & editing, J.J. and X.H.

Acknowledgments: This work was financially supported by the National Natural Science Foundation of China(Grant No. 51674270), National Major Project (Grant No. 2017ZX05009–003), Major Project of the National NaturalScience Foundation of China (No.51490650), and the Foundation for Innovative Research Groups of the NationalNatural Science Foundation of China (Grant: No. 51521063).

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Moraes, R.P.; Valera, T.S.; Pereira, A.M.C.; Demarquette, N.R.; Santos, A.M. Influence of the type of quaternaryammonium salt used in the organic treatment of montmorillonite on the properties of poly(styrene-co-butylacrylate)/layered silicate nanocomposites prepared by in situminiemulsion polymerization. J. Appl. Polym. Sci.2011, 119, 3658–3669. [CrossRef]

2. Kawasumi, M. The discovery of polymer-clay hybrids. J. Polym. Sci. Part A Polym. Chem. 2004, 42, 819–824.[CrossRef]

3. Zhang, G.; Ke, Y.; He, J.; Qin, M.; Shen, H.; Lu, S.; Xu, J. Effects of organo-modified montmorillonite on thetribology performance of bismaleimide-based nanocomposites. Mater. Des. 2015, 86, 138–145. [CrossRef]

4. EI-Sigeny, S.; Mohamed, S.K.; Taleb, M.F.A. Radiation synthesis and characterization of styrene/acrylicacid/organophilic montmorillonite hybrid nanocomposite for sorption of dyes from aqueous solutions.Polym. Compos. 2014, 35, 2353–2364. [CrossRef]

5. Huang, H.; Liu, H. Synthesis of the Raspberry-Like PS/PAN Particles with Anisotropic properties via seededemulsion polymerization initiated by γ-Ray radiation. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 5198–5205.[CrossRef]

6. Palanisamy, A. Water-blown polyurethane–clay nanocomposite foams from biopolyol—Effect of nanoclayon the properties. Polym. Compos. 2013, 34, 1306–1312. [CrossRef]

7. Chen, Y.N.; Chung, P.Y.; Yen, S.C. Conductivity and methanol permeability of sulfonated polystyrenemembrane with dispersed montmorillonite nanoclay. Polym. Compos. 2012, 33, 2105–2113. [CrossRef]

8. Dawson, J.I.; Oreffo, R.O. Clay: New opportunities for tissue regeneration and biomaterial design. Adv. Mater.2013, 25, 4069–4086. [CrossRef] [PubMed]

9. Wang, H.J.; Wang, H.Y.; Chen, K.; Song, Y.M.; Wei, Z.; Xue, M. The modification of lanthanum-exchangedmontmorillonite with anionic surfactants to enhance the thermal stability of polyvinyl chloride. J. Appl.Polym. Sci. 2015, 132. [CrossRef]

10. Huang, J.C.; Zhu, Z.K.; Yin, J.; Qian, X.F.; Sun, Y.Y. Poly(etherimide)/montmorillonite nanocompositesprepared by melt intercalation: Morphology, solvent resistance properties and thermal properties. Polymer2001, 42, 873–877. [CrossRef]

11. Ji, J.Q.; Ke, Y.C.; Pei, Y.; Zhang, G.L. Effects of highly exfoliated montmorillonite layers on the properties ofclay reinforced terpolymer nanocomposite plugging microspheres. J. Appl. Polym. Sci. 2017, 134. [CrossRef]

12. Momani, B.; Sen, M.; Endoh, M.; Wang, X.L.; Koga, T.; Winter, H.H. Temperature dependent intercalationand self–exfoliation of clay/polymer nanocomposite. Polymer 2016, 93, 204–212. [CrossRef]

Appl. Sci. 2018, 8, 964 16 of 17

13. Soulestin, J.; Rashmi, B.J.; Bourbigot, S.; Lacrampe, M.F.; Krawczak, P. Mechanical and optical properties ofpolyamide 6/clay nanocomposite cast films: Influence of the degree of exfoliation. Macromol. Mater. Eng.2012, 297, 444–454. [CrossRef]

14. Hwang, S.J.; Yong, L.J.; Lee, S.J. Properties of High-Impact Polystyrene/Organoclay NanocompositesSynthesized via In Situ Polymerization. J. Appl. Polym. Sci. 2008, 110, 1141–1450. [CrossRef]

15. Liu, L.; Wei, S.L.; Lai, X.J. In situ synthesis and characterization of polypropylene/polyvinylacetate-organophilic montmorillonite nanocomposite. J. Appl. Polym. Sci. 2012, 124, 4107–4113. [CrossRef]

16. Chen, G.M.; Liu, S.H.; Chen, S.J.; Qi, Z.N. FTIR spectra, thermal properties, and dispersibility ofa polystyrene/montmorillonite nanocomposite. Macromol. Chem. Phys. 2001, 202, 1189–1193. [CrossRef]

17. Simons, R.; Qiao, G.G.; Powell, C.E.; Bateman, S.A. Effect of surfactant architecture on the properties ofpolystyrene-montmorillinite nanocomposites. Langmuir 2010, 26, 9023–9031. [CrossRef] [PubMed]

18. Zhu, J.; Morgan, A.B.; Lamelas, F.J.; Wilkie, C.A. Fire properties of polystyrene-clay nanocomposites.Chem. Mater. 2001, 13, 3774–3780. [CrossRef]

19. Zhang, Z.; Liao, L.; Xia, Z. Ultrasound-assisted preparation and characterization of anionic surfactantsmodified montmorillonites. Appl. Clay. Sci. 2010, 50, 576–581. [CrossRef]

20. Paiva, L.B.D.; Morales, A.R.; Diaz, F.R.V. Organoclays: Properties, preparation and applications.Appl. Clay. Sci. 2008, 42, 8–24. [CrossRef]

21. Alansi, A.M.; Alkayali, W.Z.; Al-Qunaibit, M.H.; Qahtan, T.F.; Saleh, T.A. Synthesis of exfoliatedpolystyrene/anionic clay MgAl-layered double hydroxide: Structural and thermal properties. RSC Adv.2015, 5, 71441–71448. [CrossRef]

22. Zhu, L.; Guo, J.; Liu, P. Effects of length and organic modification of attapulgite nanorods onattapulgite/polystyrene nanocomposite via in-situ radical bulk polymerization. Appl. Clay. Sci. 2016,119, 87–95. [CrossRef]

23. Ianchis, R.; Donescu, D.; Petcu, C.; Ghiurea, M.; Anghel, D.F.; Stanga, G.; Marcu, A. Surfactant-free emulsionpolymerization of styrene in the presence of silylated montmorillonite. Appl. Clay. Sci. 2009, 45, 164–170.[CrossRef]

24. Qian, Z.; Zhou, H.; Xu, X.; Ding, Y.; Zhang, S.; Yang, M. Effect of the grafted silane on the dispersion andorientation of clay in polyethylene nanocomposites. Polym. Compos. 2009, 30, 1234–1242. [CrossRef]

25. Wang, L.; Wang, X.; Chen, Z.; Ma, P. Effect of doubly organo-modified vermiculite on the properties ofvermiculite/polystyrene nanocomposites. Appl. Clay. Sci. 2013, 75–76, 74–81. [CrossRef]

26. Zhang, K.; Chen, H.; Chen, X.; Chen, Z.; Cui, Z.; Yang, B. Monodisperse silica-polymer core-shellmicrospheres via surface grafting and emulsion polymerization. Macromol. Mater. Eng. 2003, 288, 380–385.[CrossRef]

27. Li, H.; Yu, Y.; Yang, Y. Synthesis of exfoliated polystyrene/montmorillonite nanocomposite by emulsionpolymerization using a zwitterion as the clay modifier. Eur. Polym. J. 2005, 41, 2016–2022. [CrossRef]

28. Wang, J.; Hu, Y.; Wang, S.; Chen, Z. Sonochemical one-directional growth of montmorillonite–polystyrenenanocomposite. Ultrason. Sonochem. 2005, 12, 165–168. [CrossRef] [PubMed]

29. Yang, W.T.; Ko, T.H.; Wang, S.C.; Shih, P.I.; Chang, M.J.; Jiang, G.J. Preparation of polystyrene/claynanocomposite by suspension and emulsion polymerization. Polym. Compos. 2010, 29, 409–414. [CrossRef]

30. Yi, D.; Yang, H.; Zhao, M.; Huang, L.; Camino, G.; Frache, A.; Yang, R. A novel, low surface charge density,anionically modified montmorillonite for polymer nanocomposites. RSC Adv. 2017, 7, 5980–5988. [CrossRef]

31. Yang, L.; Ke, Y. Synthesis of polystyrene nanolatexes via emulsion polymerization using sodium dodecylsulfonate as the emulsifier. High Perform. Polym. 2014, 26, 900–905. [CrossRef]

32. Piscitelli, F.; Posocco, P.; Toth, R.; Fermeglia, M.; Pricl, S.; Mensitieri, G.; Lavorgna, M. Sodium montmorillonitesilylation: Unexpected effect of the aminosilane chain length. J. Colloid. Interf. Sci. 2010, 351, 108–115. [CrossRef][PubMed]

33. Bhanvase, B.A.; Pinjari, D.V.; Gogate, P.R.; Sonawane, S.H.; Pandit, A.B. Synthesis of exfoliatedpoly(styrene-co-methyl methacrylate)/montmorillonite nanocomposite using ultrasound assisted in situemulsion copolymerization. Chem. Eng. J. 2012, 181–182, 770–778. [CrossRef]

34. Ezquerro, C.S.; Ric, G.I.; Miñana, C.C.; Bermejo, J.S. Characterization of montmorillonites modified withorganic divalent phosphonium cations. Appl. Clay. Sci. 2015, 111, 1–9. [CrossRef]

Appl. Sci. 2018, 8, 964 17 of 17

35. Faucheu, J.; Gauthier, C.; Chazeau, L.; Cavaille, J.Y.; Mellon, V.; Lami, E.B. Miniemulsion polymerization forsynthesis of structured clay/polymer nanocomposites: Short review and recent advances. Polymer 2010, 51, 6–17.[CrossRef]

36. Wu, Y.; Zhang, J.; Zhao, H. Functional colloidal particles stabilized by layered silicate with hydrophilic faceand hydrophobic polymer brushes. J. Polym. Sci. Polym. Chem. 2009, 47, 1535–1543. [CrossRef]

37. Hu, J.; Chen, M.; Wu, L. Organic-inorganic nanocomposites synthesized via miniemulsion polymerization.Polym. Chem. 2011, 2, 760–772. [CrossRef]

38. Greesh, N.; Sanderson, R.; Hartmann, P. Prepatation of polystyrene-clay nanocomposites via dispersionpolymerization using oligomeric styrene-montmorillonite as stabilizer. Polym. Int. 2012, 61, 834–843.[CrossRef]

39. Patel, H.A.; Somani, R.S.; Bajaj, H.C.; Jasra, R.V. Preparation and characterization of phosphoniummontmorillonite with enhanced thermal stability. Appl. Clay. Sci. 2007, 35, 194–200. [CrossRef]

40. Bee, S.L.; Abdullah, M.A.A.; Mamat, M.; Bee, S.T.; Sin, L.T.; Hui, D.; Rahmat, A.R. Characterization ofsilylated modified clay nanoparticles and its functionality in PMMA. Compos. Part B Eng. 2017, 110, 83–95.[CrossRef]

41. Periyayya, U.; Joo, K.H.; Hong, C.; Eun-Kyung, S.; Youn-Sik, L. Preparation of exfoliated high-impactpolystyrene/MMT nanocomposites via in situ polymerization under controlling viscosity of the reactionmedium. Polym. Compos. 2010, 29, 142–148.

42. Greesh, N.; Hartmann, P.C.; Cloete, V.; Sanderson, R.D. Impact of the clay organic modifier on the morphologyof polymer–clay nanocomposites prepared by in situ free-radical polymerization in emulsion. J. Polym. Sci.Polym. Chem. 2008, 46, 3619–3628. [CrossRef]

43. Dong, Z.; Liu, Z.; Zhang, J.; Han, B.; Sun, D.; Wang, Y.; Huang, Y.J. Synthesis of montmorillonite/polystyrenenanocomposites in supercritical carbon dioxide. Appl. Polym. Sci. 2004, 94, 1194–1197. [CrossRef]

44. Chen, S.; Lu, X.; Zhang, Z.; Wang, T.; Pan, F. Preparation and characterization of poly (methylmethacrylate)/reactive montmorillonite nanocomposites. Polym. Compos. 2016, 37, 2396–2403. [CrossRef]

45. Fu, X.; Qutubuddin, S. Synthesis of polystyrene–clay nanocomposites. Mater. Lett. 2000, 42, 12–15. [CrossRef]

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